Rotation Problem?

I was building a model, duplicating and then rotating objects into position, but half way through the process, my ability to rotate using the mouse stopped working. I can rotate objects that way in other scenes, but not in that file. In the file I was working in, I can still rotate via the numeric controls, but they are not very useful in this instance.

Does anyone know what went wrong or how to undo it without chucking what I’ve already done? Could I have hit some kind of hot key by accident to cause this problem?

Any help would be appreciated.

https://blenderartists.org/forum/viewtopic.php?t=30689

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That was it, Thank You.

Thanks! This SAME thing happens to me all the time, and now I know what to do!